ANSI/SCTE 108:2006 pdf download.Test Method for Dielectric Withstand of Coaxial Cable.
I .1 The purpose of this document is to provide a test standard tr detecting flaws in the insulation (sometimes referred to as the dielectric) of a completed coaxial cable. This test, usually referred to as a Hipot or Dielectric Withstand Test, verifies that the insulation can withstand a specified voltage applied between the center conductor and outer conductor for a specified time interval, without resulting in a dielectric breakdown. Upon successful completion of this hipot test. it can he concluded that the inner and outer conductors are properly insulated from each other.
1.2 Under normal operating conditions there will be a small amount of leakage current within the dielectric of any product (in this case the insulation between the center and oater conductors of a coaxial cable). However, if 2 conductors are not properly insulated from each other, the application of high voltage can cause dielectric breakdown. L)ielectric breakdown results in excessive current flow that is substantially larger than the nominal leakage current for the dielectric material being tested.
1.3 Traditionally, either an AC or DC voltage may he used for the test. The DC voltage used should be the peak of the equivalent AC (RMS) voltage, or 1.414 times the AC (RMS) voltage.
2.0 AC VS. I)C HlI’Ol’ $fl
2.1 AC and DC hipot tests have inherent advantages and disadvantages depending on the application they arc used for. Some advantages of an AC hipot test include: an AC voltage stresses a cable’s insulation in both polarities; it is unnecessary to allow the coaxial cable to discharge; it is unnecessary to apply an AC voltage gradually. Some advantages of a DC hipot test include: a DC hipot doesn’t need to have a high output current capacity: the application of a DC voltage allows the hipot to clearly display true leakage current; a DC hipot can he used to test highly capacitive products using far less power.
2.2 A hipot test is used to apply high voltage to the insulation of a coaxial cable and make sure that the leakage current produced is limited to an acceptable level. This leakage current is limited by the cable’s shunt impedance resulting from the parallel capacitance and the insulation resistance between the two conductors.
2.3 A big advantage of the DC hipot test is that the current due to the capacitance falls to zero when the cable is exposed to a constant DC voltage. Therefore, the hipot measures and displays only the true resistive leakage current. Thus the test operator can be assured that the inductive reactance isn’t coming into play when performing a DC hipot test. However, there is another current component to consider when applying a DC voltage to a coaxial cable. As the DC voltageapplied increases, there is a current needed to “charge” the capacitance of the cable up to the tes voltage. This charging current will he much higher than the actual leakage current of the cable’s insulation. So it is necessary to ramp up the DC voltage slowly so that the charging current doesn’t become so great as to cause a false failure of the hipot. Once the cable is fully charged. the charging current will fall to zero and the oniy remaining current component will be the real leakage current.
2.4 When an AC hipot is used, a capacitive current component is produced along with the resistive leakage current (the sum of which is known as the ioial current). An AC hipot test will measure and display the total current. The reactive capacitance of a coaxial cable usually is much more significant than its resistance. Thus, the approximate total current can be assumed to he a good indication of the real leakage current.
3.0 IQt1PllNT
3.1 As discussed above, the 1-lipot or Dielectric Withstand Tester is the device needed to check for flaws in the insulation of coaxial cable. With a wide range of hipo testers available, the test operator should make sure the equipment selected incorporates adequate safety provisions and be properly sized tr the voltage, trip current. and timing function capabilities for the test to he conducted. Such features include, hut are not limited to.
